In recent years, there have been vehicles such electric vehicles, hybrid vehicles and fuel-cell vehicles that employ an electric motor as a drive source for propelling the vehicle and is equipped with a battery of a large capacity storing an electric power for driving the electric motor.
Japanese Patent Laying-Open No. 2003-209969 has disclosed a power supply apparatus having a plurality of power supply stages connected, in parallel with each other, to an inverter. Each of the power supply stages includes a battery and a bidirectional boost/back converter. In this power supply apparatus, the bidirectional boost/back converter corresponding to each battery is controlled so that the batteries included in the respective power supply stages may be uniformly charged or discharged.
In the power supply apparatus disclosed in Japanese Patent Laying-Open No. 2003-209969, the charging and discharging of each battery can be controlled independently of the others. However, this power supply apparatus requires the plurality of converters and therefore is expensive.
For reducing the cost, it may be envisaged to connect the plurality of batteries to one converter. However, this structure may further complicate the control of the charging and discharging. Particularly, the control of the charging and discharging must be performed to reduce variations in temperature of the battery as far as possible because the internal resistance of the battery varies according to the temperature.
When there are variations in internal resistance between the plurality of batteries, a battery having a smaller internal resistance supplies or receives a larger power. Therefore, the temperature of this battery rises. For example, in the case where an internal resistance of a battery decreases with increase in its temperature, the variations in internal resistance further increase. Particularly, the possibility of occurrence of this tendency increases when an electric power is input or output to/from the battery at a high current rate (e.g., of about 10 C), where 1 C means a current value of the battery in the case where a theoretical battery capacity is charged or discharge in one hour.
Generally, when the battery temperature rises to a predetermined upper limit, the input/output power of the battery is restricted for protecting the battery. Therefore, even when the plurality of batteries are employed for obtaining the power supply of a large capacity, the power that can be practically input/output may become smaller than an assumed power. Accordingly, the charge/discharge must be controlled to reduce the variations in temperature between the plurality of batteries as far as possible.